Electrically Tuneable Exciton-Polaritons through Free Electron Doping in Monolayer WS2 Microcavities

Abstract

This study demonstrates control over light–matter coupling at room temperature combining a field effect transistor (FET) with a tuneable optical microcavity. This microcavity FET comprises a monolayer tungsten disulfide, WS2, semiconductor which is transferred onto a hexagonal boron nitride flake that acts as a dielectric spacer in the microcavity, and as an electric insulator in the FET. In this tuneable system, strong coupling between excitons in the monolayer WS2 and cavity photons can be tuned by controlling the cavity length, which is achieved with excellent stability, allowing to choose from the second to the fifth order of the cavity modes. Once the strong coupling regime is achieved, the oscillator strength of excitons is then modified in the semiconductor material by modifying the free electron carrier density in the conduction band of the WS2. This enables strong Coulomb repulsion between free electrons, which reduces the oscillator strength of excitons until the Rabi splitting completely disappears. The charge carrier density is controlled from 0 up to 3.2 × 1012 cm−2, and over this range the Rabi splitting varies from a maximum value that depends on the cavity mode chosen, down to zero, so the system spans the strong to weak coupling regimes.